The invention relates to improved method and device for pressing a workpiece, and more particularly to method and device for heat-pressing a workpiece such as a multi-layered board by applying heat and pressure simultaneously thereto.
A multi-layered board such as printed wiring board (PWB) is generally manufactured by heat-pressing a plurality of vertically stacked members such as plurality of circuitry members and prepreg members inserted between the respective adjacent pair of circuitry members.
Heat-pressing is generally carried out by employing a pressing device having a pair of opposing heating platens, one of which is arranged to be driven toward and away from the other one.
Practically, a stack of circuitry members and prepreg members is prepared and placed between the pair of heating platens to be heat-pressed therebetween. During heat-pressing process, the temperature of the heating platens is controlled such that the resin component of the prepreg members is substantially liquidized for a predetermined period of time to thereby bond the adjacent circuitry members and unite all the stacked members together to form a multi-layered printed wiring board.
As a printed wiring board manufactured as above is required to have uniform thickness, heat-pressing must be carried out with heating platens having high surface flatness. On the other hand, a printed wiring board is recently becoming thinner and thinner due to the demand for producing compact electric devices in which a printed wiring board is to be installed. For example, the thickness of a printed wiring board to be manufactured is less than 100 xcexcm, and in this case, a heating platen must have the surface flatness variation of less than 10 xcexcm.
If the surfaces of the heating platens do not satisfy the condition mentioned above and have relatively large undulation, the pressure exerted from the heating platens on the printed wiring board varies across the printed wiring board. Such variance in the pressure results in thickness variance of the pressed printed wiring board. In addition, poor bonding property between the circuitry members occurs at locations where the pressure was insufficiently applied.
In addition to the above, pressing devices have recently employed large heating platens in order to simultaneously press a plurality of printed wiring boards by one pressing operation. However, since the realization of the required flatness of the heating platens becomes more difficult with the size thereof, the number of defective printed wiring boards, i.e. boards having low thickness uniformity and/or poor bonding property, also increases.
In order to avoid the above defects, it has been proposed to employ a cushion member, to be placed between a workpiece and a heating platen, such as a kraft paper that has a small surface roughness and small deformation on one side of which hardly affects the surface roughness of the other side.
Materials used in such a cushion member, however, have extremely low thermal conductivity compared to the heating platen. Such low thermal conductivity requires longer heating time and results in low productivity.
It is therefore an object of the invention to provide improved method and device for pressing a workpiece capable of applying uniform pressure over a workpiece irrespective the level of flatness of a pressing surface.
It is another object of the invention to provide improved method and device for pressing a workpiece capable of applying uniform pressure over a workpiece irrespective the size of a pressing surface.
It is a further object of the invention to provide improved method and device for heat-pressing a workpiece capable of applying uniform pressure over a workpiece as well as of effectively heating a workpiece.
According to an aspect of the invention, a method for pressing a workpiece with a uniform pressure across the workpiece is provided. In this method, a pressing plate is located on the pressing surface of a platen. Then a spacer thicker or deeper than the workpiece is placed beside the workpiece. The spacer is pressed by the platen to define a gap between the pressing plate and the workpiece. Then, a liquid layer formed between the platen and the pressing plate is pressurized such that the pressing plate bends toward the workpiece due to the pressure of the liquid layer and thereby presses the workpiece. Since the liquid layer exerts a uniform pressure over the pressing plate, the pressing plate applies a uniform pressure over the workpiece.
Thus, the platen can keep contacting the spacer and the distance between the platen and the workpiece remains constant while the liquid is being pressurized, which prevents the pressing plate from breaking due to too much bending toward the workpiece.
According to another aspect of the invention, a pressing method is provided in which a workpiece to be pressed is located on the pressing surface of the platen and a cushion member is provided between the platen and the workpiece. The cushion member includes a pair of plates joined to each other to form a sealed space therebetween. The sealed space is filled with liquid to form a liquid layer.
Then, the platen is forced towards the workpiece such that the workpiece is pressed by the cushion member. Since the cushion member is filled with liquid, the cushion member applies a uniform pressure across the workpiece.
Optionally, an intermediate plate may be provided between the cushion member and the workpiece before the workpiece is pressed by the cushion member. The intermediate plate may be bendable along the surface of the workpiece when the intermediate plate is pressed against the workpiece.
Alternatively, a stack of workpiece may be formed by alternatively pilling a plurality of the workpieces and the intermediate plates and placed the pressing surface of the platen for pressing.
The intermediate plates mentioned above assists in enlarging the area that the cushion member can exert uniform pressure on the workpiece.
According to another aspect of the invention, there is provided a device for pressing a workpiece, which has a platen movable towards the workpiece and a pressing plate provided between the platen and the workpiece such that the workpiece is pressed by the pressing plate when the platen is moved toward the workpiece.
A liquid layer is formed between the pressing plate and the platen. This liquid layer transfers the force for pressing the workpiece from the platen to the pressing plate. During the transmission of the force, the force exerted on the liquid layer from the platen is changed to a constant pressure of the liquid layer irrespective the surface geometry of the platen. Therefore, the force exerted on the pressing plate by the liquid layer is constant across the pressing plate and, as a result, the pressure applied on the workpiece from the pressing plate becomes uniform all over the workpiece.
The pressing plate is a thin plate so that it bends along the surface of the workpiece during pressing. Optionally, the workpiece side of the pressing plate is finished in a mirror-smooth state for smooth contact between the pressing plate and the workpiece.
In some cases, the platen is a heating platen heated to a temperature sufficient to heat the workpiece to a temperature required for heat-pressing. In such case, the liquid layer is preferably a layer of heat conductivity oil. Since the heat transfer oil has high heat conductivity, the heat of the heating platen is effectively transferred to the workpiece. Therefore the workpiece can be heated in a short time and thus the time required for the whole pressing process is reduced.
Optionally, the pressing plate is detachably mounted to the platen such that a space is defined therebetween. The space is utilized to form the liquid layer by filling the space with liquid. The pressure of the liquid filled in the space is preferably controlled by a pressure controller such that, for example, the pressure of the liquid is raised only when the workpiece is pressed.
In the above case, the press may further include a spacer for keeping the distance between the workpiece and the platen during the pressing. The spacer is arranged to the workpiece side of the platen and sustains the platen forced towards the workpiece such that a clearance is defined between the pressing plate and the workpiece when the liquid layer is not pressurized. The clearance is defined to have a size small enough to allow the pressing plate to bend by the liquid layer and press the workpiece when the liquid layer is pressurized.
The pressure controller raises the pressure of the liquid such that the pressing plate bends and presses the workpiece only when the platen is sustained by the spacer. Further, the pressure controller raises the pressure of the liquid up to a predetermined level that is not greater than the maximum pressure the platen can apply. The pressing plate bends towards the workpiece when the liquid is pressurized. However, as long as the conditions mentioned above are satisfied, the bend of the pressing plate is restricted to a small degree due to the small clearance and the breakage of the pressing plate caused by high pressure of the liquid layer is prevented.
In some cases, the pressing device includes a pressure controller adapted to control the pressure of the liquid layer, and a backing plate joined to the pressing plate such that a space is defined therebetween. The space is filled with liquid to form the liquid layer. The backing plate is detachably mounted to the platen so that it can be exchanged together with the pressing plate joined thereto. The backing plate includes a conduit that is in communication with the space and an opening formed at a side surface of the backing plate. The opening is connected to the pressure controller so that the pressure of the liquid layer can be controlled.
In the above case, the pressing device may be configured such that the space is formed when the liquid is pressurized to a predetermined level, while the space disappears and the pressing plate becomes in contact with the backing plate when the pressure of the liquid is reduced below the predetermined level.
Optionally, a groove may be formed to the backing plate at the surface facing the pressing plate, which is in communication with the conduit to provide the liquid into the groove. The groove may be formed in a grid pattern extending over the surface of the backing plate so that the liquid quickly spreads over the backing plate and the liquid layer is formed in a short time after the liquid layer is pressurized.
Where the platen is a heating platen, the backing plate is mounted to the platen such that the surface facing the platen is in contact therewith so that the heat of the heating platen is efficiently conducted to the workpiece.
In some cases, the pressing device includes a pair of the platens, a pair of the pressing plates, and a pair of the liquid layers. The platens, pressing plates and liquid layers are arranged to sandwich and press the workpiece between the pair of pressing plates so that a uniform pressure can be applied over both sides of the workpiece.
In some cases, the pressing device comprises a cushion member which carries the pressing plate and an additional plate. The additional plate and the pressing plate are joined to each other to define a sealed space therebetween. The sealed space is filled with liquid to form the liquid layer. The cushion member is located between the platen and the workpiece such that the additional plate faces the platen and the pressing plate faces the workpiece. During the press, pressure is applied from the platen to the workpiece through the cushion member. Since the cushion member includes the liquid layer, uniform pressure is applied across the workpiece even if the surface flatness of the platen is relatively low.
Where the platen is a heating platen, the sealed space of the cushion member is filled with heat transfer oil so that the heat of the heating platen is efficiently conducted to the workpiece during the press.
In the above case, it is preferable to form the pressing plate and the additional plate from a material of high heat conductivity such as metal. It is also preferable that the workpiece side of the pressing plate is of a mirror finished surface so that the pressing plate does not scratch the workpiece during pressing.
The sealed space may be formed by welding the pressing plate and the additional plate along the entire peripheries thereof.
Alternatively, the cushion member may include a spacer located between the pressing plate and the additional plate to keep the pressing plate substantially flat and thereby increase the area that can be utilized to press the workpiece. The spacer may also serve to keep the additional plate substantially flat. The spacer may further serve to keep the pressing plate and the additional plate substantially parallel to each other. The spacer above may be an annular member having an upper rim and a lower rim. The upper and lower rims may be respectively welded to the pressing plate and the additional plate to form the sealed space.
The pressing device of the above mentioned case may include a pair of the above described platens, and a pair of the above described cushion members. These platens and cushion members may be arranged to sandwich and press the workpiece between the pair of cushion members.
Optionally, the press may include a flat intermediate plate. During pressing of the workpiece, the intermediate plate is located to one of first and second places, where the first place is defined between the workpiece and the cushion member and the second place is defined between two workpieces stacked to be pressed simultaneously. Such intermediate plate located to the first or second place assists in applying uniform pressure to the workpiece.
Preferably, the intermediate plate is thin enough to bend along the surface of the workpiece when it presses the workpiece. The intermediate plate is preferably made of a material of high heat conductivity such as metal.